The present invention relates to a varistor material for a surge arrester, to a process for preparing said varistor material, and to the use of said varistor material for a surge arrester having a target switching field strength ranging from 250 to 400 V/mm.
Varistors, i.e. current/voltage non-linear resistors, are well known in the art and are frequently used as overvoltage protection devices. In general, overvoltage protection devices are employed in power systems or circuits of electronic equipment to protect the power system or electronic equipment by removing overvoltage superimposed on the normal voltage. The basis for this overvoltage protection is based on the varistor's characteristic to function as an insulator at normal voltage, but to show a low resistance when overvoltage is applied.
Among the known varistors, discs of a ZnO based varistor material are widely used. Apart from the main component ZnO, these varistor materials usually comprise many other additives which have an influence on the varistor's characteristics.
In this regard, the highly non-linear characteristics of varistor materials are mainly attributed to the presence of Bi2O3, which forms monolayers of Bi atoms around ZnO grains and creates potential barriers, and also of transition metals, such as Co, Mn and others, which stabilize the potential barriers by creating additional defects at the grain boundary. Conventional varistor materials further comprise Sb2O3 or SiO2 in concentrations of some mol-% for microstructural control by forming so-called “spinels” that inhibit grain growth.
In addition to the ZnO phase, the intergranular bismuth oxide phase and the spinel phase, a fourth phase called pyrochlore phase of the nominal formula Bi3Sb3Zn2O14, which might include also other dopants in minor concentrations, is usually present. According to Inada et al, Japanese Journal of Applied Physics, 1980, Vol. 19, No. 3, pp. 409 to 419, for example, the pyrochlore phase starts to appear during sintering at temperatures of about 650 to 750° C. (depending on the actual composition) and disappears at temperatures above 950 to 1050° C., where spinel is formed out of pyrochlore. Nevertheless, varistors sintered even at a temperature above 1200° C. generally have a pyrochlore phase to a certain extent, due to a reformation during relatively slow cooling rates existing in production. According to the mentioned report by Inada, the pyrochlore is assumed to play no role in the nonohmic property.
Desired properties of good varistor materials are a well-defined switching voltage VS or switching field strength ES, respectively, a high non-linearity coefficient α (alpha) in the switching region—according to equation I=(V/C)α (alpha) a high energy uptake, low power losses and a high stability during lifetime.
The switching voltage VS is approximately 3 volts per grain boundary and depends on the total number of grain boundaries in series, and therefore also on the number of varistor discs in series and on the block size. The switching field strength ES is a material property and is determined by the grain size of the material or the density of grain boundaries, respectively. In the following, ES is defined as the switching field strength at a current density of 0.1 mA/cm2.
Most of the commercially available varistor materials have a switching field strength in the range 150-250 V/mm. Hence, varistors with such a switching field strength can be designated “normal field varistors” or “medium field varistors”. Consequently, varistors with a switching field strength below 150 V/mm are designated “low field varistors”, and varistors with a switching field strength above 250 V/mm are designated “high field varistors”. In the following, an additional differentiation between the expression “high field varistor” (ES=250-400 V/mm) and “extra high field varistor” (ES>400 V/mm) is made.
A high field varistor material is of special interest for high voltage arresters (or “surge arresters”), since it allows reducing its dimensions. Providing such a high field varistor material is however very challenging, mainly due to its thermal management.
On one hand, the power losses during normal continuous operation of a high field varistor are generated in a smaller volume, causing the varistor disc and parts of the arrester housing to run at elevated temperatures. Higher temperatures are often not desired or acceptable, due to the resulting ageing of all the involved materials. High temperatures at normal operating conditions also reduce the capability of the varistor to absorb thermal load during an overvoltage pulse and add a risk for thermal runaway after such loading conditions. Therefore, significantly lower specific power losses (power losses normalized to the volume and the applied field strength) for high field varistor materials are required compared to varistor materials with normal switching field strength.
On the other hand, thermal loading during an overvoltage pulse is also more severe because of the smaller volume. Therefore also excellent impulse performance is needed, leading to the requirements of high electrical non-linearity in the high current region and high energy absorption capability.
In addition, degradation of electrical properties over lifetime must be avoided.
To reach high or extra high field strengths, the grain size of the varistor material has to be significantly reduced.
Several possibilities to influence the grain size and hence the switching field strength are known. One option is the reduction of the sintering temperature—as for example referred to in U.S. Pat. No. 4,719,064—with the result of smaller grains and a higher switching field strength. This is however only possible within a certain range, since lower sintering temperatures usually lead to lower non-linearity and downgraded impulse performance. With regard to the decrease in the non-linearity coefficient by lowering the top temperature and shortening the dwell time during sintering, the effect is for example shown in Balzer et al, J. Am. Ceram. Soc., vol. 87, No. 10 (2004), pp. 1932. An additional disadvantage of a too low sintering temperature lies in the fact that thereby the material does often not densify properly during sintering, leaving a porous structure, which could reduce the energy absorption capability.
Another option to adjust the electrical properties of varistor materials is by changing the chemical composition.
Of the components contained in conventional varistor materials, Bi2O3 is the only component forming liquid phase during sintering. The reduction of Bi2O3 content therefore reduces the amount of liquid phase during sintering, slowing down grain growth, and thus also increasing the switching field strength. However, the effect of increasing the switching field strength by the reduction of bismuth is relatively small. In addition, more bismuth is necessary for smaller grains to cover the grain boundaries and to form stable potential barriers. Otherwise the non-linearity gets heavily reduced.
As mentioned above, Sb2O3 is often used in conventional varistor materials for controlling the microstructure and for increasing the switching field. Antimony is known to form zinc antimony spinels at an early stage of sintering that hinder grain growth. The addition of Sb2O3 is for example disclosed in EP-A-0961300. As will be pointed out in detail below, Sb2O3 has however recently been found to have a negative impact on the non-linearity and the power loss of the varistor material due to an absorption of a part of the available bismuth oxide.
It has further been proposed to add silicon in the range of 0.1 to several mol-% for reaching a higher field strength. In this regard it is for example referred to U.S. Pat. No. 5,107,242, according to which silicon oxide is used in an amount of 0.6 to 2.0 mol-%, said silicon oxide precipitating in the grain boundary layer to prevent development of ZnO grains. The use of silicon is further taught in EP-A-0 320 196, U.S. Pat. No. 4,920,328, U.S. Pat. No. 4,719,064, DE-A-2739848 and U.S. Pat. No. 5,075,666.
The silicon addition provokes the formation of zinc silicon spinels with a similar effect as antimony. However, the increase of the switching field strength for compositions with high silicon content is often tremendous, leading not only to a high field varistor material but to an extra high field varistor material.